1. Field of the Invention
This invention relates to the field of elastomer-based elastic connectors which filter vibrations between a frame element of a motor vehicle, to which a suspension is fastened, and the rigid frame or body of the vehicle. Such an elastic connector enables the rigidity of the elastic connection to be modified as a function of conditions.
2. Background Information
The automobile industry is continuously attempting to improve the quality of the insulation of the passenger compartment against vibrations caused by the undercarriage, while simultaneously trying to improve the precision of the steering to thereby improve "road handling". In order to accomplish these improvements, new parts are being installed on modern vehicles to provide antivibration elasticity between the vehicle and the undercarriage, or "ground link".
However, the low rigidity that is required in all directions for the effective filtering of vibrations is generally not compatible with the powerful forces that can be transmitted statically and dynamically by the wheels. In particular, a rear suspension which is mounted elastically, i.e. with a certain flexibility, introduces, during turns, micro-deflections and changes in direction, which the designer must overcome by an appropriate modification of the kinematics, ensuring the wheel clearance.
As disclosed in European Patent No. 0 028 692 to Porsche, significant geometric deformations of the elastic filtering parts are acceptable, since their effect is offset in the trajectory, geometrically imposed by the kinematics of the deformable connections.
European Patent No. 0 252 805 to Automobiles Peugeot & Automobiles Citroen, proposes a rear suspension which has a pre-stressed transverse elastic connection, which connection, beyond a certain lateral acceleration threshold, rotates around an imaginary point. This rotation about an imaginary point induces an understeer effect caused by the rear suspension of the vehicle. This solution requires, as a result of the variations caused by the corresponding trajectory, the sudden change from one state of elastic equilibrium to another.
Another family of improvements for connection devices utilizes the change from a situation where the vibrations are effectively filtered in the presence of low forces, to another filtering situation which is significantly more rigid, in which the elastic parts absorb major steering forces by means of a rapid loss of the initial flexibility over a short stroke, which is translated by a veritable stop function, the disadvantage of which is a loss of filtering.
Examples of parts with rapidly variable rigidity are disclosed in French Patent 898 055 to Getefo, which patent explains the theory of operation, and French Patent 2 453 746 to General Motors, which illustrates the application of the theory for various elastically mounted suspension systems on vehicles.
The change from a situation in which the vibrations are effectively filtered to another situation where there is a less flexible connection therefore constitutes a compromise solution sought by automotive designers. However, such a compromise solution would essentially only provide effective filtering under certain conditions.
A third type of connecting device provides priority filtering in essentially two directions, including the essentially vertical direction bearing the principal load. This solution, however has the disadvantage that it achieves elastic connections in the two filtered directions, which connections are much more rigid than any connections in a third direction. Practically equivalent realizations of connecting devices are also proposed by French Patent No. 2 621 664 to Hutchinson and French Patent No. 2 622 660 to Caoutchouc Manufacture et Plastiques, which corresponds to U.S. Pat. No. 4,854,562. The former discloses the use of a rod which is elastically articulated at its extremities, and the latter discloses a metal cable under tension, and embedded in an elastomer support. These two devices provide a high monodirectional rigidity, combined with a good level of filtering in any other direction.
A fourth type of connecting device, which operates under a totally different concept, is proposed by the use of a so-called "active" device which modifies the rigidities as a function of current conditions.
European Patent 0 290 328 (Regie Nationale des Usines Renault) recommends varying the natural rigidity of suspension elements, in particular of the motor mount, by using a shape-memory alloy embedded in the elastomer. The alloy can be heated by the passage of an electric current through the alloy to vary the rigidity of the device from one degree of rigidity to another. Such a device however can not provide instantaneous reaction since heating by means of electric control is typically slow and therefore the connection is unable to achieve an immediate response.
An analysis of the prior art apparently shows that the known elastic connection solutions are based on the passive displacement of the elastic system as a consequence of the accelerations applied. This displacement is translated by a time lag between the application of the force and the initiation, in reaction, of the appropriate rigidity. It therefore seems that the prior art does not include a device which reacts directly to the accelerations exerted, most particularly the lateral accelerations, and thereby participates in the immediate control of the trajectory of the vehicle.